Field of the Invention
The present invention relates to a liquid crystal display device including ultra-thin glass and a method for manufacturing the liquid crystal display device.
Description of the Background Art
In recent years, there has been proposed a liquid crystal display for use in a curved form (curved display) or a liquid crystal display device capable of displaying a dual screen (dual-screen display) in which a parallax barrier is arranged on a display surface of a liquid crystal panel. In these liquid crystal displays, ultra-thin glass is commonly used. For example, Japanese Patent Application Laid-Open No. 2003-337550 mentions a liquid crystal panel including, as ultra-thin glass, a glass substrate having an ultra-thin thickness of about 0.01 to 0.15 mm, in order to achieve a flexibly bendable liquid crystal panel that can be also used in a curved display. Japanese Patent Application Laid-Open No. 2011-128547 discloses: a liquid crystal display device capable of the dual-screen display in which, as ultra-thin glass, a glass substrate having an ultra-thin thickness of about 0.1 mm or 0.3 mm is provided only at one substrate side of the liquid crystal display device; and a method for manufacturing the liquid crystal display device. In a liquid crystal display device adapted for the dual-screen display or curved display, ultra-thin glass having a substrate thickness of about 0.1 mm is used. In a process for manufacturing such a liquid crystal display device, after at least one of two glass substrates is thinned, forming a cell substrate by bonding the two substrate with a seal is difficult for strength reasons. Accordingly, as described in the above-mentioned Patent Documents, after two substrates are bonded to each other with a seal to form a cell substrate, a step is performed in which at least one of the substrates is thinned by means of polishing or etching to achieve ultra-thin glass.
Firstly, in a method for manufacturing the liquid crystal panel for the curved display disclosed in Japanese Patent Application Laid-Open No. 2003-337550, a thinning step for thinning a cell substrate in which a liquid crystal is encapsulated is performed after the cell substrate is obtained by performing a sealing step of sealing the liquid crystal by means of a seal surrounding the liquid crystal and two glass substrates, that is, by performing a so-called one drop filling method. In a case of adopting the one drop filling method to encapsulate the liquid crystal, designing of a light-shielding layer and wirings involves a restriction necessary for irradiating the seal with light so that the seal is promptly cured, in order to prevent contamination of the liquid crystal which may be caused by a contact of an uncured seal with the liquid crystal. The restriction particularly leads to thinning of the light-shielding layer and wirings. Thus, in a liquid crystal panel for the curved display, a larger stress is applied to the light-shielding layer and wirings than usual. Therefore, there is a fear that cracking may occur in the light-shielding layer or breaking may occur in the wirings. Such a problem can be avoided by adopting, as a method for encapsulating the liquid crystal, the vacuum injection method instead of the one drop filling method. That is, in a state of a cell substrate in which mother substrates are bonded to each other by a seal, one glass substrate is thinned to achieve ultra-thin glass, then the substrates are cut into a size corresponding to an individual liquid crystal panel, and then a liquid crystal is injected and sealed in a vacuum state.
On the other hand, in a method for manufacturing the liquid crystal panel for the dual-screen display, a step of forming a parallax barrier on a surface of the ultra-thin glass obtained as a result of the thinning is further performed. In this parallax barrier formation step, a light-shielding metal film is formed by means of sputtering or the like, and therefore the substrate is heated so that the substrate temperature rises to a temperature that gives not a little influence on an organic material. Thus, in a case of performing the step of forming a parallax barrier on the cell substrate in which the liquid crystal has been encapsulated by the one drop filling method as shown in the method disclosed in Japanese Patent Application Laid-Open No. 2003-337550, the liquid crystal that is an organic material causes a quality alteration. Accordingly, in the method for manufacturing the liquid crystal panel for the dual-screen display disclosed in Japanese Patent Application Laid-Open No. 2011-128547, such a problem is avoided by adopting the vacuum injection method as the method for encapsulating the liquid crystal. That is, in a state of the cell substrate in which mother substrates are bonded to each other with a seal and no liquid crystal is encapsulated, one glass substrate is thinned to achieve ultra-thin glass, then a parallax barrier is formed, then the substrates are cut into a size corresponding to an individual liquid crystal panel, and then a liquid crystal is injected and sealed in a vacuum state.
As described above, some of problems occurring in the display device including ultra-thin glass, such as the curved display or the dual-screen display, can be avoided by adopting a manufacturing process in which the step of thinning a glass substrate to achieve ultra-thin glass, the step of cutting into a size corresponding to an individual liquid crystal panel, and the step of injecting and sealing a liquid crystal in a vacuum state, which is a liquid crystal injection method using a so-called vacuum injection method, are sequentially performed. However, even in a case of using this method, the following problems remain unsolved.
Firstly, a first problem will be described. When vacuuming of the interior of a cell and a liquid crystal injection step of injecting a liquid crystal from a liquid crystal injection port are performed under a state where one of substrates is ultra-thin glass, a gap between the substrates is properly held by balancing among the pressure of the liquid crystal drawn into the cell, the atmospheric pressure in the outside of the cell at a time of exposure to the atmosphere, the repulsive force exerted by spacers that keep the gap between the two substrates, and the tensile force of the substrates between the spacers that are dispersedly arranged in a plane. However, since the substrate made of the ultra-thin glass has a weak tensile force, a portion thereof located between the spacers and not held by the spacers is overwhelmed by pressing from the atmospheric pressure. Therefore, at a location corresponding to this portion, the gap between the substrates is narrower than the predetermined inter-substrate gap, and depending on conditions, the gap completely disappears. Additionally, at a location where the liquid crystal has been drawn, the pressure of the liquid crystal acts as a resistive force (reactive force) against the pressing from the atmospheric pressure, thereby preventing the disappearance of the gap.
In performing the liquid crystal injection step of injecting a liquid crystal from the liquid crystal injection port, encapsulation of the liquid crystal starts from a portion near the injection port, and the interior of the cell is sequentially filled with the liquid crystal, until a portion thereof farthest from the injection port (and more specifically, corner portions thereof located at both ends of the side opposed to the side where the injection port is provided) is filled. Accordingly, in these corner portions located on the side opposed to the side where the injection port is provided, which are finally filled with the liquid crystal, the inter-substrate gap is narrowed by the pressing from the atmospheric pressure before these corner portions are filled with the liquid crystal. This narrowing delays completion of the encapsulation of the liquid crystal. If the pressure in the outside of the cell gradually rises and completely returns to the atmospheric pressure under a state where the completion of the encapsulation of the liquid crystal is delayed, a portion of the spacer located at a corresponding position may be deformed beyond an assumed amount of deformation. As a result, the deformation exceeds a range of elastic deformation which is a reversible change, and causes plastic deformation, or even worse, the spacer is fully destroyed. Once a reversible change range is exceeded like this, it is no longer possible to keep a proper gap between the substrates even when the liquid crystal is put therein with a delay. Additionally, in the portion of the spacer that has been fully destroyed, a pillar that supports the cell is lost. Therefore, in any case, unevenness of the gap occurs. Moreover, when the fully destroyed spacer is dispersed to reach a display region within the cell, a display failure due to an abnormal alignment occurs. Furthermore, there is a fear that a constituent element of the spacer of the destroyed spacer may, as an impurity, run into the liquid crystal and contaminate the liquid crystal, which results in a deterioration in the reliability.
Then, a second problem will be described. In a case of the dual-screen display, a parallax barrier formation step, and particularly, a light-shielding layer formation step in which the substrates are heated, is additionally performed before the liquid crystal injection step shown in the above-described the first problem is performed. When the substrates are heated, adoption of the vacuum injection method avoids heating the liquid crystal, but resins of columnar spacers and an alignment film that are arranged within the cell are exposed to the heat treatment. Since the resins of the columnar spacers and the alignment film are also made of an organic material, not a little alteration is caused in the quality of the material. To be specific, examples of a caused change include an increase in the amount of gas emission and a reduction in the range of elastic deformation of the columnar spacers. If the vacuum injection step of injecting the liquid crystal is performed under a state where such a change is caused, depending on an increase in the amount of gas emission, a gas coming from a surface of the substrate accumulates within the cell, which makes it difficult to achieve a normal liquid crystal injection even though normal vacuuming is performed. As a result, more time is taken to complete the encapsulation of the liquid crystal in the above-mentioned corner portions located farther from the injection port. Moreover, since the range of elastic deformation of the columnar spacers is narrowed, it is likely to cause plastic deformation and destruction, which are unrecoverable deformation. Furthermore, these plastic deformation and destruction are likely to occur also in pressure application that is performed in order to push out an extra liquid crystal as a time of sealing the injection port. That is, occurrence of the above-described first problem becomes more conspicuous.
As thus far described, in the liquid crystal display device including ultra-thin glass, such as the curved display or the dual-screen display, some of problems can be avoided by adopting a liquid crystal injection method using a so-called vacuum injection method. On the other hand, as described above, unevenness of the gap and a display failure due to an abnormal alignment occur in the corner portions that are farthest from the injection port. Thus, in the conventional display device including ultra-thin glass, such as the curved display or the dual-screen display, and in the manufacturing of the conventional display device, an ideal structure and an ideal manufacturing method that do not cause the above-described problems have not been proposed yet.